This invention is generally directed to photoresponsive imaging members, and more specifically the present invention is directed to improved photoresponsive imaging members containing as hole transporting substances high molecular weight polysilylene compositions. In one important embodiment of the present invention, there is provided a layered photoresponsive imaging member comprised of a polysilylene hole transporting compound wherein the lower molecular weight fractions thereof are removed, and a photogenerating layer, which members are particularly useful in liquid development imaging processes. Further, there is provided in one particular aspect of the present invention an improved layered photoresponsive imaging member comprised of a supporting substrate, a photogenerating layer, and in contact therewith a hole transport layer comprised of a polysilylene compound, especially poly(methylphenyl silylene), poly(n-propylmethyl-co-cyclohexylmethyl silylene), and other similar polysilylenes with a weight average molecular weight of from about 400,000 to about 1,000,000; and wherein there are removed therefrom low molecular weight fractions such as those with a molecular weight of from about 4,000 to about 40,000. With further respect to the aforementioned imaging members, the polysilylene hole transporting compound layer can be located as the top layer of the imaging member, or alternatively may be situated between the supporting substrate and the photogenerating layer. Moreover, the present invention relates to the use of the improved imaging members of the present invention in electrophotographic, and especially xerographic, imaging processes including those wherein liquid and dry developer compositions are selected for rendering the images formulated visible.
The formation and development of electrostatic latent images on the imaging surfaces of photoconductive materials by electrostatic means is well known. The photoreceptor selected may comprise a conductive substrate containing on its surface a layer or layers of photoconductive insulating materials, and in many instances there can be used a thin barrier layer between the substrate and the photoconductive layer to prevent charge injection from the substrate into the photoconductive layer upon charging. Numerous different photoconductive members for use in xerography are known, including for example a homogenerous layer of a single material such as vitreous selenium, or composite layered imaging members with a photoconductive compound dispersed in other substances. An example of one type of composite photoconductive layer used in xerography is described, for example, in U.S. Pat. No. 3,121,006 wherein there is disclosed a number of layers comprising finely divided particles of photoconductive inorganic compounds dispersed in an electrically insulating organic resin binder.
There are also known photoreceptor materials comprised of other inorganic or organic materials wherein the charge carrier generation and charge carrier transport functions are accomplished by discrete contiguous layers. Additionally, photoreceptor materials are disclosed in the prior art which includes an overcoating layer of an electrically insulating polymeric material, and in conjunction with this overcoated type photoreceptor there have been proposed a number of imaging methods. However, the art of xerography continues to advance and more stringent demands need to be met by the copying apparatus to permit an increase in performance standards especially with liquid developer compositions, and to permit higher quality images. The photoconductive imaging member of the present invention represents such an improved member, and has other advantages as disclosed hereinafter.
Recently, there have been developed layered photoresponsive imaging members, including those comprised of generating layers and transport layers as disclosed in U.S. Pat. No. 4,265,990, and overcoated photoresponsive materials with a hole injecting layer overcoated with a transport layer, followed by an overcoating of a photogenerating layer and a top coating of an insulating organic resin, reference U.S. Pat. No. 4,251,612. Examples of photogenerating layers disclosed in these patents include trigonal selenium and metal, or metal free phthalocyanines. illustrative examples of the transport compounds that may be employed are comprised of certain aromatic amines as mentioned therein. The disclosures of each of these patents, namely U.S. Pat. Nos. 4,265,990 and 4,251,612, are totally incorporated herein by reference. The U.S. Pat. No. 4,265,990 patent is of particular interest in that it discloses layered photoresponsive imaging members similar to those illustrated in the present application with the exception that the hole transporting substances of this patent are comprised of aryl amine compositions, while in accordance with the present invention the hole transporting substance is a specific polysilylene.
Many other patents are in existence describing photoresponsive imaging members including layered imaging members with generating substances such as U.S. Pat. No. 3,041,167, which describes an electrophotographic imaging member with an overcoated imaging member containing a conductive substrate, a photoconductive insulating layer, and an overcoating layer of an electrically insulating polymeric material. This member is utilized in an electrophotographic copying method by, for example, initially charging the member with an electrostatic charge of a first polarity, and imagewise exposing to form an electrostatic latent image which can be subsequently developed to form a visible image.
In U.S. Pat. No. 3,041,116 there is disclosed a photoconductive material with a transparent plastic material overcoated on a layer of vitreous selenium, which is present on a recording substrate. Apparently, in operation the free surface of the transparent plastic is electrostatically charged to a desired polarity, followed by exposing the imaging member to activating radiation, which generates a hole electron pair in the photoconductive layer, and wherein the electrons move to the plastic layer and neutralize the positive charges contained on the free surface of the plastic layer, thus creating an electrostatic image. Also, there is disclosed in U.S. Pat. Nos. 4,232,102 and 4,233,383, the disclosures of which are totally incorporated herein by reference, the use of sodium carbonate doped and barium carbonate doped photoresponsive imaging members containing trigonal selenium. Other representative patents disclosing layered photoresponsive imaging members include 4,115,116; 4,047,949 and 4,081,274.
In addition, there are also known layered photoresponsive imaging members wherein there are selected various squaraine compounds, reference for example U.S. Pat. Nos. 4,552,822; 4,415,639; 4,471,041; and 4,486,520, the disclosures of each of these patents being totally incorporated herein by reference.
There is also illustrated in U.S. Pat. No. 4,618,551, the disclosure of which is totally incorporated herein by reference, photoresponsive imaging members similar to those described in the present application with the primary exception that the photoresponsive imaging members of the present invention contain therein specific polysilylenes of an average molecular weight of about 400,000 to about 1,000,000, and wherein the low molecular weight fractions thereof of, for example, from about 4,000 to about 50,000, are removed. More specifically, there is illustrated in the aforementioned patent a polysilylene hole transporting compound for use in imaging members, which compound is of the formula as illustrated in claim 1 with specific examples of polysilylenes being poly(methylphenyl silylene) of an average molecular weight of greater than 50,000. Additionally, there is disclosure presented in this patent wherein the molecular weight of the polysilylene is from about 300,000 to about 800,000. However, these polysilylenes also contain therein lower molecular weight fractions, that is from about 4,000 to about 50,000, which low molecular weight fractions are believed to cause the resulting imaging members to be less resistant to liquid developers.
Moreover, the imaging members with the polysilylenes of the '551 patent are susceptible to cracking with usage which adversely effects image quality, and depending on the extent of cracking no images whatsoever may be generated. In addition, the polysilylenes of the '551 patent when exposed to liquid ink vehicles, for example subsequent to immersing the layered imaging member with the aforementioned polysilylene in Isopar L, the imaging member is susceptible to some cracking. In contrast, the imaging members of the present invention with the low molecular weight fractions removed, and particularly polysilylenes with a weight average molecular weight of from about 400,000 to about 1,000,000 with substantially no molecular weight fractions present with a weight average molecular weight of from about 4,000 to about 40,000, will not crack when immersed in Isopar L; and these imaging members possess the other advantages indicated herein.
Illustrated in U.S. Pat. No. 4,588,801 are polysilylene positive photoresist materials and processes for the preparation thereof. More specifically, it is indicated in column 11, beginning at line 23, of this patent that the polysilylenes are prepared by polymerizing halosilylenes, preferably dichloro silylenes, in the presence of an alkali metal catalyst, preferably sodium, and an inert solvent such as toluene at elevated temperatures of, for example, from 90.degree. to 100.degree. C. and under reflux; also note the disclosure in column 12, Example 1, lines 21, through column 13, line 15. A similar teaching is presented in U.S. Pat. Nos. 4,587,205 and 4,464,460.
Although imaging members with various hole transporting substances are suitable for intended purposes, there continues to be a need for improved members, particularly layered members which are comprised of specific polysilylenes; and which members are substantially completely resistant to liquid developer compositions. Moreover, there continues to be a need for specific layered imaging members which not only generate acceptable images, but which can be repeatedly used in a number of imaging cycles without deterioration thereof from the machine environment or surrounding conditions. Additionally, there continues to be a need for improved layered imaging members wherein the materials employed for the respective layers, particularly the hole transporting layer, are substantially inert to the users of these members. Further, there continues to a need for improved photoresponsive imaging members which can be prepared with a minimum number of processing steps, and wherein the layers are sufficiently adhered to one another to allow the continuous use of these imaging members in repetitive imaging processes. Also, there continues to be a need for new hole transporting compounds that are also useful as protective overcoating layers, and as interface materials for various imaging members. Furthermore, there is a need for hole transporting polysilylene compositions that may be useful as binder polymers for photogenerating substances comprised of organic materials. There also is a need for new hole transporting substances which enable increased mobility of holes in layered imaging members. Likewise, there is a need for hole transporting compounds with increased stability, for example, wherein there is no extraction of these compounds from the layered imaging members in which they are incorporated when, for instance, liquid developers are selected for rendering the latent electrostatic latent image visible. Furthermore, there is a need for hole transporting compounds useful in layered imaging members, which compounds are superior insulators in the dark compared to may other known hole transporting compounds, thus enabling charging of the resulting imaging member to higher fields while maintaining cyclic stability, and allowing improved developability. Also, there is a need for imaging members with new hole transporting compounds which can function as resinous binders. Additionally, there is a need for enabling the preparation of imaging members with new hole transporting compounds, wherein the preparation allows for the selection of a variety of solvents, inclusive of toluene, benzene, tetrahydrofuran, cyclohexane, and halogenated solvents in addition to methylene chloride.
Moreover, there is a need for layered photoresponsive imaging members with polysilylenes wherein low molecular weight fractions of from about 4,000 to about 40,000 have been removed thereby enabling members that are resistant to cracking and can be selected for electrophotographic imaging processes, especially wherein liquid developer compositions are utilized. There is also a need for flexible layered imaging members with polysilylenes of a weight average molecular weight of from about 400,000 to 1,000,000, and wherein the ratio of the weight average molecular weight to the number average molecular weight is from about 1.3 to about 3.0.